In sliding surfaces of a lubricated sliding pairing it is essential for an amount of sliding friction and also for the service life of the sliding pairing in particular of the sliding bearing that a sufficient amount of lubricant is provided in all operating conditions with an even distribution between the contact surfaces of the sliding pairing. Thus, in particular a beginning of a relative movement between the two sliding surfaces is critical.
An increasing use of start-stop installations in motor vehicles greatly emphasizes the importance of this beginning of the relative movement for bearings of a crank shaft since the number of start-up processes of the sliding bearings is increased by a factor of one hundred or more.
Therefore the contact surfaces of sliding surfaces, in particular of sliding bearings are machined so that they include microscopically small indentations which are used as a reservoir for lubricant. These indentations are provided due to a natural roughness of the material of the sliding surface or they are introduced in a controlled manner. Therefore the contact portion of a sliding bearing, thus the surface portion with which the contact surfaces are actually in contact each other is always significantly below 100%, partially even below 60%.
The respective structuring of the sliding surfaces is achieved by special machining steps like grinding, finishing, or honing wherein, however, a particular arrangement of the indentations cannot be predetermined and also the variation with respect to size, in particular depth of these indentations is relatively large. In particular the result of the structuring is also highly dependent on the experience of the person performing the process.
In order to obtain a defined structuring of the contact surface of the sliding bearing with respect to number, size, depth and distribution of the indentations it is also known to impinge the surface with a laser in order to achieve the desired indentations.
This method, however, is very time consuming for a large number of indentations and the laser beam generates an undesirable mound which envelops the indentation in a ring shape and the laser processing leads to undesirable new hardness zones.
Furthermore a machining method of electro chemical milling is known (ECM) which can also be used in a pulsed manner (PECM).
This way three dimensional surfaces are fabricated, for example the described indentations in surfaces wherein typically only a removal of 30 μm at the most is economically viable when performed by this method.
Approaching an accordingly configured electrode with a negative contour towards the surface to be processed which acts as another electrode removes material from the surface in the form of ions.
For current conduction and removing dissolved materials a current conducting liquid is pressed through the gap between tool and work piece during the entire process.
When the work pieces are crankshafts, in particular crankshafts for car engines with a high number of cylinders an additional disadvantage is that these crankshafts are instable during processing and thus difficult to position and that it is also difficult to structure these work pieces.
When the work pieces are crank shafts, in particular crank shafts for car engines with a high number of cylinders there is the additional factor that these are work pieces that are instable during machining and thus difficult to position and therefore the structuring is also difficult.
Dimensional precision of a finished crankshaft is primarily determined by assessing the following parameters in addition to maximum bearing width:                Diameter deviation=maximum deviation from a predetermined nominal diameter of the bearing pin,        Circularity=macroscopic deviation from a circular nominal contour of the bearing pin determined by a distance of an outer enveloping circle and an inner enveloping circle,        Concentricity=radial dimensional deviation for a rotating work piece caused by a eccentricity of the rotating bearing and/or a shape deviation of the bearing from an ideal circular shape        Roughness defined by the mean single depth of roughness Rz=a value representing the microscopic roughness of the bearing defined by the sum of the highest profile peak and the lowest profile valley averaged over five single measuring ranges,        Roughness defined by the arithmetic mean rough value Ra=arithmetic average of the absolute value of the coordinate value of the roughness profile inside one of said single measuring range,        Roughness defined by the reduced peak height Rpk=height of the triangle having the same surface area as the summit area surfaces, wherein the triangle has a certain base length of a S-like Abott-curve; this value allows evaluation of the peak areas of a surface profile,        Contact percentage=contacting surface portion of the microscopic surface structure which is in contact with a contacting opposite surface, and additionally for crank pin bearings:        Stroke deviation=dimensional deviation of the actual stroke (distance of an actual center of the crank pin from the actual center of the crank journal) from the nominal stroke, and        Angular deviation in degrees or stroke related longitudinal deviation in circumferential deviation of the actual angular position of the crank pin from its nominal angular position relative to the central bearing axis and with respect to the angular position to the remaining crank pins.        
Thus, maintaining the desired tolerances for these parameters is limited by the available machining methods and also by the instability of the work piece and the machining forces.
Efficiency and economics of a processing method are of great importance for practical applications, in particular for series production where cycle time and thus production cost is of great importance, whereas these limitations do not apply for a processing of test samples of prototypes.
This applies in particular for the last process steps when manufacturing for example a crankshaft, finishing and surface structuring, in particular of their bearings.
In this context it is known as a matter of principle to use the ECM method for obtaining a particular low roughness as described for example in DE 10 2008 011 893 and also from DE 10 2004 027 89.